Human Immunodeficiency Virus (HIV) affects millions of people worldwide, and the prevention of HIV remains a very high priority, even in an era of widespread antiretroviral treatment. In the United States, the Center for Disease Control (CDC) estimates that of all HIV-positive US residents, approximately one fifth are unaware of their status, and this small proportion is responsible for transmitting half the new infections each year [2]. Worldwide, the gap in prompt diagnosis and treatment is far greater. At the end of 2010, an estimated 34 million people were living with HIV worldwide, up 17% from 2001. Although the majority of new HIV infections continue to occur in sub-Saharan Africa, the CDC estimated that the annual incidence of HIV infection from 2008-2011 in the United States has remained stable at around 15-16/100,000, with over 40,000 new infections each year. Thus, it is an urgent global health priority to find a safe and potent HIV vaccine that would prevent HIV infection or blunt its initial impact prior to diagnosis, including both destruction of the gut CD4 pool [3] and high risk of transmission [4].
A fully efficacious vaccine is anticipated to be able to elicit both potent cellular responses and broadly neutralizing antibodies capable of neutralizing HIV-1 variants from different clades.
Moreover, a recent clinical study indicates that non-neutralizing Env-specific antibodies may have some protective capacity that is linked to subtype-specific antibody function [9]. Broadly neutralizing antibodies are directed against highly conserved regions in the viral envelope. Until recently, most anti-HIV vaccines used purified HIV antigenic proteins, such as gp160, gp41 or gp120 presented in a soluble form. Most envelope (Env) protein-based immunogens are monomeric envelope molecules that elicit binding antibodies, but not potent neutralizing antibodies. This is in part due to the fact that neutralizing antibodies recognize tertiary and quaternary epitopes on the native, trimeric structure of the viral envelope proteins. In addition, most monomeric Env-based immunogens do not induce a cell-mediated response. It was reported that stabilized trimers of HIV-1 Env induced broadly neutralizing antisera against HIV-1 in vivo. See, e.g., US 2012/0045472.
Live attenuated vaccines have proven to be highly efficacious in humans and in non-human primates (NHP) against certain viral diseases, such as a live attenuated simian immunodeficiency virus (SIV) based vaccine for preventing SIV infection. Unfortunately, due to safety risks associated with live attenuated HIV, such a strategy is not applicable for HIV human vaccine.
In order to elicit both potent cellular responses and broadly neutralizing antibodies, recombinant vectors have been used to express genes for HIV antigenic proteins in vivo as an alternative to live attenuated viral vaccines. The use of replication incompetent recombinant viral vectors has been explored for vaccines and other types of gene therapy. In particular, replication incompetent recombinant adenoviral vectors, particularly adenovirus serotypes 2 and 5 (Ad2 and Ad5) have been extensively studied for gene delivery applications, including vaccination. Although such replication incompetent Ad5 vector-based vaccines have been shown to elicit protective immune responses in a variety of animal models, the utility of recombinant Ad5 vector-based vaccines for HIV and other pathogens can be limited by the high seroprevalence of Ad5-specific neutralizing antibodies (NAbs) in human populations [17]. For example, in a seroepidemiology study of 4,381 subjects worldwide, it was observed that Ad5 NAb titers were nearly universal and high titer in sub-Saharan Africa, with the majority of individuals exhibiting Ad5 NAb titers >200 [14].
Several HIV-1 vaccine efficacy trials have been conducted using vaccines based on recombinant Ad5 vector-based vaccines. These studies include the HVTN 502/STEP (Merck Ad5), HVTN 503/Phambili (Merck Ad5), and HVTN 505 (NIH VRC DNA/Ad5) HIV-1 vaccine efficacy trials. However, all three of these HIV-1 vaccine efficacy studies, which utilized nonreplicating Ad5 and DNA/Ad5 vaccines, showed no efficacy against HIV-1 infection. Moreover, a trend towards increased HIV-1 infection was observed in subjects vaccinated with the Merck Ad5 vaccine from the STEP study as compared with placebo. Experience to date with replication incompetent vectors such as adenovirus subtype 5 for HIV vaccine has been disappointing, with failure to show benefit in several efficacy trials [5-8].
Concerns regarding the safety of Ad5 vectors, particularly from the STEP study [8, 10], have led to the exploration of biologically substantially different Ad vectors from alternative serotypes as viral vaccine vectors [11-13]. One example of an alternative adenovirus serotype to Ad5 is Adenovirus serotype 26 (Ad26). Ad26 is a relatively uncommon virus in humans, and is not known to replicate in any other species. A number of surveys for adenovirus in different populations have shown it to be isolated only rarely, and even when isolated, seldom associated with symptoms. Experimental immunization, likewise, showed little evidence for serious infection. See, e.g., references [14], and [27]-[43]. Thus, there is no evidence from observational studies that Ad26 causes clinical symptoms in healthy adults, and experimental data from an Ad26 challenge study also suggested that enteric Ad26 infection does not produce symptoms [44]. Replication-defective adenovirus vectors, rAd26, can be grown to high titers in Ad5 E1-complementing cell lines suitable for manufacturing these vectors at a large scale and at clinical grade [11], and this vector has been shown to induce humoral and cell-mediated immune responses in prime-boost vaccine strategies [11, 21]. Another alternative is rAd35, a replication-defective adenovirus vector derived from Adenovirus serotype 35. The rAd35 vectors grow to high titers on cell lines suitable for production of clinical-grade vaccines [61], and have been formulated for injection as well as stable inhalable powder [62].
These alternative adenovirus vectors show efficient transduction of human dendritic cells [63, 22], and thus have the capability to mediate high level antigen delivery and presentation.
In terms of at least receptor usage, in vivo tropism, interactions with dendritic cells, innate immune profiles, adaptive immune phenotypes, and protective efficacy against SIV in rhesus monkeys, Ad26 has proven to be biologically very different from Ad5 [11, 12, 15, 19-22]. Moreover, the safety and immunogenicity of nonreplicating Ad26 vector in humans have been demonstrated (ClinicalTrials.Gov NCT01215149). Furthermore, many of the advantageous biological differences between Ad5 and Ad26, such as lower seroprevalance and low neutralizing antibody titers in humans are also present between Ad5 and Ad35.
Modified Vaccinia Ankara (MVA) virus, a replication-deficient strain of vaccinia virus, has also been used as a viral vector for recombinant expression of HIV antigenic proteins. See, e.g., US20110159036, U.S. Pat. No. 8,197,825, etc. MVA is related to Vaccinia virus, a member of the genera Orthopoxvirus in the family of Poxviridae. Poxviruses are known to be good inducers of CD8 T cell responses because of their intracytoplasmic expression. However, they are generally believed to be poor at generating CD4 MHC class II restricted T cells. See, e.g., [64].
One possible drawback of replication-incompetent viral vectors is that expression of the target gene to be delivered to the host from the viral vector can decrease following administration of the vector. Being unable to replicate or propagate in the host, the viral vector cannot produce any new copies that can subsequently be used to augment gene expression, thus requiring re-administration of the viral vector. If the same adenovirus serotype is re-administered to the host, the host can generate neutralizing antibodies to that particular adenovirus serotype, resulting in a serotype specific anti-adenovirus response. Such a serotype specific anti-adenovirus response can prevent effective re-administration of the viral vector, rendering it less effective as a vaccine or gene delivery vehicle.
Accordingly, there is a need in the art for improved vaccines that can be used to induce a protective immunity against HIV infection. Such a vaccine preferably would be simple to administer, long-acting, and have minimal adverse effects. It further would preferably be effective against a wide diversity of circulating types of HIV transmission, including the most frequent for multiple regions of the world.